Users are asked give details including an email and home address in order to register for the course. Registration can then take up to seven days to complete, according to the website.

Deputy Assistant Commissioner Lucy D’Orsi, senior national co-ordinator for protective security at the NPCC, said the course was especially useful for anyone working in or regularly visiting crowded places.

“The threat level remains at substantial, meaning an attack is likely, so giving everyone the chance to be extra eyes and ears for police and local security teams helps to keep all communities safe,” she said.

“The festive period is obviously a very busy one, so this is a good time to join up and become a CT Citizen.”

She said more than 1.5 million modules have been completed to date.

The UK’s terrorism threat level is currently substantial, meaning an attack is likely. The grading was lowered from severe – meaning an attack is highly likely – on 4 November.